Nondestructive downhole seismic vibrator source and processes of utilizing the vibrator to obtain information about geologic formations

ABSTRACT

The invention relates to a nondestructive downhole seismic source capable of generating S V  -waves, S H  -waves, and P-waves alone or in combination to determine information about a surrounding geologic formation. The invention also includes processes of performing crosswell tomography and reverse vertical seismic profiling. The invention also includes a means and process to carry out in hole seismic logging operations.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to the application entitled DOWNHOLEELECTROMAGNETIC SEISMIC SOURCE by BjORN N. P. PAULSSON filedconcurrently herewith and assigned U.S. Ser. No. 841,073 and filed Mar.18, 1986. Said application is completely incorporated herein byreference for all purposes.

FIELD OF THE INVENTION

This invention relates to downhole seismic sources. More specifically,this invention relates to nondestructive hydraulic downhole seismicsources and processes of obtaining information about undergroundformations penetrated by at least one wellbore.

BACKGROUND OF THE INVENTION

During drilling or after the completion of a well, it is often desirableto obtain information about the formation surrounding the wellbore. Inwells completed into or through zones in the formation containinghydrocarbons, it is desirable to have techniques of obtaininginformation about these producing zones. Well logs are often run todetermine such parameters as resistivity, conductivity and otherparameters from which the properties of the oil and the producingformation can be deduced to give a clearer picture of the environmentsurrounding the wellbore.

One measurement technique involves the use of geophones which arelowered into the wellbore while surface seismic sources generate wavesto pass through the geologic formation. The geophones sense these waves.Subsequent processing of the recordings derived from these waves providea clearer picture of the environment surrounding the wellbore. However,the weathering layer of the earth's crust attenuates a great deal of theenergy from the seismic sources before it reaches the zones of interestin the formation surrounding the wellbore. Providing a seismic sourcewithin a wellbore below the weathering layer and spaced from a differentwellbore in which the geophones are located would remove the effects ofthe weathering layer.

Typically, a well is completed into a formation by cementing a linerwithin the wellbore. Any source used within the wellbore must be capableof imparting the desired amount of energy into the formation to bedetected either at the surface or in an adjacent wellbore. However, theform of the energy must not input shear or compressive borehole stressescapable of separating the liner from the cement or casing. Thesestresses should be less than the maximum recommended shear stress on thecasing cement interface, which is about 20 psi, as specified by theAmerican Petroleum Institute (API RP2A Oct. 22, 1984).

FIG. 12 and FIG. 13 are graphs comparing the borehole stresses andeffective energies between known destructive downhole sources, namely, a1.1 lb (500 gm) dynamite charge, a 40 cu. inch (655 cc) airgun, and a4,000 lbf output (18,000 newtons) seismic source of the invention. Thevalues are applicable around 100 hz, a frequency which is within theusual frequency band. The figures show the seismic vibratory sourceachieves high effective energies with low borehole stresses. The 10 psiborehole stress induced by the seismic source of the invention is onlyhalf of the recommended maximum induced shear stress on the casingcement interface according to American Petroleum Institute standards(API RP2A Oct. 22, 1984).

Currently, a technique called "Vertical Seismic Profiling", (VSP), isused to obtain information about particular zones of interest within theformation surrounding the wellbore. Typically, this technique requireslowering a geophone into a wellbore while providing various surfaceseismic energy sources, such as dynamite, to impart seismic energy intothe formation to be received by the geophone. The geophone is lowered toa specific depth within the wellbore and a surface seismic energy sourceis located on the surface to impart the seismic energy into theformation which is detected by the geophone and recorded. The geophoneis moved to a different depth and the process is repeated. The surfaceseismic energy source is moved to various locations at various offsetsfrom the wellbore and the process is repeated. This process is extremelycostly and time-consuming. The repeated use of destructive surfacesources renders this process unsuitable for use in populated areas andunpopular with many surface landowners.

A great deal of the time and expense could be eliminated if anondestructive seismic source could be placed in a wellbore to be usedin a reverse VSP (RVSP) process. The RVSP process requires a source in awellbore and numerous geophones on the surface. The seismic energygenerated from the seismic source is detected by the geophones on thesurface. Since numerous geophones can be laid out in a two-dimensionalarray from the wellbore, the location of the geophones can remainconstant while the seismic source moves instead of requiring movementboth of the seismic source and the geophones. Assuming a nondestructiveseismic source can be located within a wellbore spaced from an adjacentwellbore containing one or more geophones, superior crosswell tomographyprocesses of production wells can be performed because the source can belocated below the attenuating weathering layer of the earth's crust. Instudying the subterranean formations, it would be desirable to receiveand analyze a host of different types of seismic waves, e.g., P-waves,S_(V) -waves, and S_(H) -waves. Thus, it would be highly desirable tohave an apparatus capable of nondestructively generating one or more ofthese types of waves within a wellbore for reception by geophoneslocated either at the surface or in an adjacent wellbore to deduceinformation about the formation. It would also be desirable to have asource which can nondestructively generate frequencies in excess of 100Hz below the weathering layer to perform crosswell tomography andcrosswell profiling. Crosswell tomography between adjacent wellborescannot be adequately performed at the present time using oil wellsbecause present available downhole seismic sources, e.g., dynamite orthe air gun are all impulsive forces generating shear stress far inexcess of API standards for detectable surface seismic waves. Attemptsto tomographically image reservoirs using surface data gatheringsources, e.g., geophones, have failed due to the attenuation andfiltering of the higher frequencies by the weathering layer, as well asunfavorable source-receiver configuration.

SUMMARY OF THE INVENTION

I have invented a nondestructive downhole seismic source capable ofgenerating detectable seismic waves, that is, waves which can bedetected by sensing devices positioned in the wellbore itself, on thesurface, or in an adjacent wellbore. The source can generate P-waves,S_(V) -waves, and S_(H) -waves alone or in combination to perform RVSP,crosswell tomography, and other useful operations for determininginformation about the formation. The seismic source can also be used torecord seismic in-hole logs by recording the seismic signal using areceiver in the same well as the downhole vibrator. The receiver shouldbe spaced a distance from the seismic source clamping means but attachedto the seismic source. An attenuating spacer will be between the sourceand the receiver to dampen the vibrations traveling along the toolcontaining the source and receiver. Preferably the nondestructivedownhole seismic source generates not only lower frequencies but alsofrequencies in excess of 100 Hz and upwards of 500 Hz or higher. As thefrequency of the seismic waves generated increases, the resolution ofany measurements increases. Thus properties of a thinner zone within theformation can be analyzed. This would enable the production engineers todevelop specific techniques to maximize the output of hydrocarbonsbetween producing zones and/or extend the life of the producing zones.Additionally, the seismic source can achieve non-destructive outputforces of up to 4,000 lbf (18,000 newtons) from a reaction mass of onlyabout 300 lbs (136 kg). This power is orders of magnitude greater thanheretofore available from existing downhole vibratory sources such asair driven vibrators. Other impulsive sources with high seismic poweroutput have tended to damage the wellbore.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a downhole seismic source within acutaway view of a wellbore;

FIG. 2 illustrates an alternative embodiment of a downhole seismicsource within a wellbore;

FIG. 3 illustrates a top view of the downhole seismic source of FIG. 2within a wellbore;

FIG. 4 illustrates a further alternative embodiment of a downholeseismic source within a cutaway view of a wellbore;

FIG. 5 illustrates a top view of the embodiment of FIG. 4;

FIG. 6 illustrates still another alternative embodiment of a downholeseismic source within a cutaway view of a wellbore;

FIG. 7 illustrates a graph of the force output of a downhole hydraulicseismic source illustrated in FIG. 2;

FIG. 8 illustrates the generation of seismic S_(V) -waves and P-waves ofa downhole seismic source;

FIG. 9 illustrates an alternative seismic wave generated by the downholeseismic source;

FIG. 10 illustrates three-dimensional reverse vertical seismic profilingusing the downhole seismic source; and

FIG. 11 illustrates crosswell tomography using a downhole seismicsource.

FIG. 12 is a graph comparing borehole stresses between downhole sourcesof dynamite, airgun, and 4,000 lbf. seismic source of the invention; and

FIG. 13 is a graph comparing effective energies between downhole sourcesof dynamite, airgun and vibrator source of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The various process and apparatus embodiments of the invention will bemore clearly illustrated by referring to the Figures. FIG. 1 illustratesa downhole seismic source 10 within an earth formation 100 penetrated bya wellbore 500. The seismic source 10 is lowered into the wellbore 500with a cable 29 attached to a suitable anchoring means 28.

The seismic source 10 includes an upper body 12 incorporating a suitableclamping means to retain the source securely in place at a predeterminedlocation within the wellbore. The clamping means should generate enoughforce to interlock the source 10 within the casing or wellbore withoutbreaking wellbore, casing, or liner or damaging the cement. The clampingmeans should generate a force which is at least greater than the forceoutput generated by the seismic source during operation. This is usuallyabout twice the generated vibrating force but in any case less than theforce necessary to fracture or break the casing or the wellbore liner ordamage the cement. A suitable clamping means in the source 10 isillustrated as a hydraulic clamping means comprising the hydraulicpistons 14a and 14b connected together by a plate 16. During theoperation of the clamping means, a hydraulic line 18, which is part ofcable 29, is pressurized from the surface which forces the pistons 14aand 14b out of their liner and the engages plate 16 with the wellbore500.

A hydraulic actuator means 22 is attached to the body 12 by any suitablemeans such as bolts, welds and the like. The piston 26 in the actuator22 is capable of moving a reaction mass 24 attached thereto parallelwith the wellbore to generate S_(V) -seismic waves in the horizontaldirection illustrated in FIG. 8 and also P-waves from the vertical tosub-vertical (0-45 from vertical). The descriptive directions (e.g.,horizontal, vertical) apply to the typical vertically-oriented well. Themovement of the reaction mass 24 to generate seismic waves isillustrated as 1000. The hydraulic actuator 22 is powered by hydraulicfluid from the surface through line 20. The size of mass and length ofstroke are a function of the desired amplitude and frequency of theseismic wave to be generated.

It is anticipated that actuator 22 could be adapted to be capable ofmoving a reaction mass 24 attached thereto in a motion perpendicular tothe wellbore to generate horizontal compressional P-waves (asillustrated in FIG. 9) and vertical shear waves (S_(V) -waves) from thevertical to sub-vertical (0-45 from vertical). The movement of thereaction mass 24 to generate seismic waves is illustrated in FIG. 9 as2000.

Alternatively, apparatus 10 can be operated so as to generate a seismicwave by engaging the clamping plate 16 with the wellbore 500 through thepressurizing of the pistons 14a and 14b. The varying of clamping forcesexerted by the pistons 14a and b in a swept frequency mode generateshorizontal compressional P-waves (as illustrated in FIG. 9) and verticalshear waves (S_(V) -waves) from the vertical to sub-vertical (0°-45°from vertical). The movement of the pistons to generate the seismicwaves is illustrated as 2000. Though seismic waves can be generated byimpacting the wellbore 500 with clamping plate 16, the preferred methodhas the plate 16 contacting the wellbore 500 at a force necessary tomaintain the source at a fixed position. Thereafter, additional pressureis exerted on the wellbore through plate 16 in a pulsating, constant oroscillatory manner so as to generate the desired seismic wave into theformation 100. Of course, the strength and magnitude of the seismic wave2000 is limited by the desire of not fracturing the wellbore casing orformation.

FIGS. 2 and 3 illustrate an alternative seismic source 30. The source 30contains a housing or body 32 which includes two hydraulically actuatedpistons 34a and 34b attached to a clamping plate 36 for positioning thesource 30 within a wellbore 500. In this embodiment, an electrical motordriving a hydraulic pump illustrated as 38 is in communication withpistons 34a and 34b. A signal from the surface, passing through anelectrical line 54 in the cable 58, causes the valve in the hydraulicpump 38 to move hydraulic pistons 34a and 34b. The movement of thepistons 34a and 34b forces the clamping means 36 into communication withthe wellbore 500. A suitable electric motor and hydraulic pump means isavailable from Vicker's Inc., Jackson, Miss., or Moog, Inc., Buffalo,N.Y. Any pump motor combination which can operate at 100° F. andgenerate 5,000 psi pressure or higher at a 5 gpm flow of hydraulic fluidis suitable in the invention.

The body 32 further contains a motion sensing device 50 in communicationwith surface electronics so that the movement of the device within thewellbore during operation can be measured. Standard accelerometers andgeophones used in wellbore environments are suitable for thisapplication.

Connected to the base of the body 32 is an electrically controlledhydraulic actuator 40 capable of moving a reaction mass 46 through themovement of a hydraulically driven piston 44 in a linear accelerator 42.A suitable source for a hydraulic actuator linear acceleratorcombination is manufactured by the Moog Corporation, Buffalo, N.Y. Thisactuator-accelerator combination is capable of imparting a wide range ofvarying frequencies to reaction mass 46. These are controlled from thesurface by an electronic signal sent through wire 56. The wires 54 and56 are electrical portions of a cable 58 which has sufficient strengthto lower and raise the source 30 within the wellbore 500. Cable 58 isattached to source 30 at cable anchor means 52.

The reaction mass 46 should additionally provide for a space to containa motion sensing device 48 (such as an accelerometer) to provide a baseline measurement for the frequencies generated during operation.

A top plan view of the source 30 positioned in the wellbore isillustrated in FIG. 3. The edges of the body 32 have serrated orcrenelated engaging portions 32a and 32b to position the body 32securely within a wellbore 500 when clamping plate 36 pressurized. Theclamping plate 36 may also have a serrated surface 36a. The serrations36a, 32a and 32b generate very high point forces when clamping plate 36contacts the wellbore 500 thereby eliminating or minimizing slippage,effectively making the clamp a mechanically stiff device.

The embodiment illustrated as 30 in FIG. 2 possesses the additionalbenefits of not requiring hydraulic lines from the surface. Source 30 iselectrically powered from the surface and has a self-contained hydraulicpower supply means necessary to reciprocate the reaction mass 46 andengage the clamping means with the wellbore. This embodiment avoids thevery high pressure losses encountered in long hydraulic lines when asource is positioned in a deep well and the hydraulic power is providedfrom the surface. By shortening the hydraulics, i.e., making theapparatus self-contained, fluid friction losses which occur in longhydraulic lines are avoided and higher forces at higher frequencies canbe generated by reciprocating the reaction mass 46 and moving the plate36. The embodiment of FIG. 1 having hydraulic lines from the surface mayprove more convenient for shallow applications.

The reaction force frequency ranges for 25-lb. (11 kg) and 75-lb. (34kg) reaction mass are illustrated in FIG. 7 for the apparatus describedin FIGS. 2 and 3. The curves represent the vibration of the reactionmasses parallel with the wellbore 500 to generate S-waves illustrated bythe movement 1000 in FIG. 2.

FIGS. 4 and 5 illustrate a preferred embodiment of the invention. Thedownhole seismic source is illustrated as 60. The source 60 has a body62 with four clamping plates 66a, 66b, 66c, and 66d, which aresymmetrically and radially positioned out from the source. The plates66a-d are engaged to meet the wellbore 500 through pistons 64a-h therebyforming a means for clamping source 60 securely to the wellbore 500. Theclamping means allows transfer of seismic S_(V), S_(H) and radialP-waves from source 60 into the formation. In a preferred pulsatingmode, the pistons 64a-h may be simultaneously pulsated radially, varyingthe force applied to the wellbore 500, generating seismic waves out intothe formation. Of course, the source 60 can be preferably designed withonly three clamps if it proves advantageous to centralize throughtriangulation. An advantage of this device is that it can generateradial seismic waves by oscillating the force applied to the formationthrough the plates in an omni-directional fashion to avoid the problemsof uni-directional seismic wave generation. The device having one ormore clamps can operate to perform the processes of the presentinvention.

The device functions as follows. A signal is sent from the surface viacable 58 to an electric motor 67 which powers a hydraulic pump 68. Aservo control valve 69 (servovalve) actuates the pistons 64a-h viahydraulic line 72 and the piston actuator valve 74. The piston actuatorvalve 74 also controls the piston 64a-h functions which generate radialseismic waves through the pulsating of opposite pairs of pistons. Theservovalve 69 directs the hydraulic fluid to either the verticalactuator 70 or rotational actuator 71 connected to the reaction mass 73.A motion sensing device 50 (such as an accelerometer) monitors theoutput of the seismic source 60. Suitable sources for the electric motorand the hydraulic pump combination include Reda Pump, Bartlesville,Okla. and Vickers, Inc. The servovalve and actuators along with thehydraulic pistons are available from Moog, Inc.

The downhole seismic source 60 also includes motion sensing devices 79which are secured to the wellbore wall 500 by expandable boot 78.Suitable sensing devices 79 include geophones and accelerometersdesigned to detect vibration of the wellbore wall 500. The sensingdevices 79 are isolated from the rest of the seismic source by anacoustic isolator 75 but connected to the device 60 by electric wires76. The wires 76 terminate in a dewar module 77 which contains theelectronics for sensing devices 79. A suitable source for the expandableboot 78 is Baker Tool of Houston, Texas.

The other motion sensing devices 50 included within the seismic source60 are not isolated from the seismic source generating portion of theapparatus. By comparing signals from the isolated sensing devices 79 andthe nonisolated devices 50, information may be obtained on the clampingeffectiveness and the energy transmission into the formation. Anelectronics package 77 receives command signals from the surface andcontrols the various components of the seismic source. The electronicspackage also receives signals from the motion sensing devices 79 and 50and transmits them to the surface. It is preferred that the electronicspackage 77 be isolated from the vibrations of actuator 60 by theacoustic isolator 75 to avoid vibration damage to sensitive electronicinstruments. Further vibrational protection may be achieved bypositioning the electronics package 77 below the boot 78 and motionsensing devices 79.

In this preferred embodiment, the source 60 is held in place by aseparate clamping means which is distinct from the clamping means in theprevious embodiments. The separation of the seismic source generationand the clamping function into distinct units permits the secureclamping of the seismic source 60 within the wellbore 500 without thepotential slipping of the seismic source 60 as the seismic waves aregenerated. A secure clamping action is generated by the metal to metalcontact of a driving wedge 82 connected to a hydraulically actuatedtwo-way piston 81 within the body portion 80 of the clamping means. Asthe driving wedge 82 moves down, it forces contact wedges 83a and 83binto the wellbore 500 when driving wedge 82 moves up, it retractscontact wedges 83a and b from the wellbore 500. The metal to metalcontact of the driving wedge 82 with the contact wedges 83a and 83bcreates a more secure clamping action than hydraulic fluid displacementwithin a piston cylinder. Contact wedges 83a and 83b may also be springloaded so they retract into the source 60 when not securing the source60 within the wellbore 500 (helpful for example if the tool experiencesa power failure). Of course, this wedge clamping means can beincorporated into any of the other embodiments if more secure clampingis desired.

The wedge clamp design of FIG. 4 may be particularly preferred when themotion of actuator vibration is in the same direction as the clampactuators as in the case of a perpendicularly oriented actuator. Thefluid filled pistons of the clamp actuators act as a shock absorberwhich distort or absorb vibrational energy intended to be transmittedinto the formation. A driving mechanism such as hydraulic piston 81 or amechanical screwdrive (not shown) drives a driving wedge 82 parallel tothe borehole 500 which in turn drives contact wedges 83a and 83b whichare dovetail to the driving wedge 82 out into contact with the wellbore500. Such a wedge clamp design does not depend solely on hydraulicpressure in the direction of vibratory motion to secure the tool.

This preferred embodiment operates according to the following process.The device 60 of the invention will be able to generate radial,torsional, or vertical forces, one at a time. The reaction mass 73serves as a load to both the vertical 70 and the rotational (torsional)71 servoactuators. To achieve the size limitation, the reaction mass maybe constructed of a heavy material such as Kennertium®. A 300 lb (136kg) reaction mass of Kennertium® would be approximately 3.5 inches (9cm) in diameter and 45 inches (114 cm) long. When activated, thevertical servoactuator 70 above the mass 73 drives the mass 73 up anddown axially. The forces generated to move the mass 73 are reactedthrough the device housing 62, through the clamp 64/66 and to thewellbore wall 500. The same reaction mass 73 is attached to the rotaryservoactuator 71. When activated, the rotary servoactuator 71 drives thereaction mass 73 torsionally.

Radial forces are preferably generated independent of the reaction mass.Linear servoactuators operating simultaneously and acting directly onthe well pipe wall generate the radial forces. These actuators arelocated below the mass and act radially and are spaced symmetricallyapart from each other. As illustrated in FIG. 5, four clamps 64/66 arespaced at 90° from each other. A three clamp version (not shown) wouldhave clamps spaced at 120°. These orientations provide multidirectionalforce output.

Each axis of the force generation requires appropriate electronics todrive the corresponding servovalve and provide closed loop servoactuatorcontrol for that axis. The vertical servoactuator is proposed as aposition control servo. A linear variable displacement transducer (LVDTposition transducer) located at the bottom of the reaction mass providesmass position feedback for the axial servo. A sinusoidal positioncommand to the vertical servoactuator will result in mass position,velocity and acceleration waveforms that are also sinusoidal.

The rotary servoactuator is also proposed as a position control servo. Arotary LVDT position transducer located at the bottom of the reactionmass provides angular position feedback for the rotary servoactuator.Prior to cycling the rotary axis a constant position command is appliedto the vertical position loop to hold the mass at some nominal positionoff the axial end stops. A sinusoidal angular position command to therotary servoactuator will then result in mass angular position, velocityand acceleration wave forms that are also sinusoidal. The sinusoidalacceleration is reacted at the well pipe wall through the clamp with atorque equal to the product of the mass rotational inertia and theangular acceleration.

The radial servoactuators are proposed as a pressure control servo. Asingle 3-way operating servovalve drives the control side of each of thethree actuators. A pressure transducer sensing this common controlpressure provides pressure feedback for the radial servoactuator. Priorto cycling the radial axis a nominal pressure command is applied to loadthe radial actuators via pads against the well pipe wall. A sinusoidalpressure command signal superimposed on the nominal loading commandsignal will result in a sinusoidally varying force applied radially tothe well pipe wall. The absolute force at each of the actuator pads isequal to the product of the control pressure and the net area of eachactuator. The radial actuators are retracted by means of a detentedspring return attached to the center of each of the pads. For asinusoidal position command signal the mass position will be given byx_(m) SIN (ωt). x_(m) is the peak mass displacement and ω is the radianfrequency of vibration. The mass velocity will be given by x_(m) (ω) COS(ωt). The mass acceleration will be -x_(m) (ω₂) SIN (ωt). The resultingoutput force is then

    F=m(x.sub.m)(ω.sup.2) SIN (ωt).

Limits on maximum force output as a function of frequency are imposed bylimited stroke, supply pressure, servovalve flow and servovalvedynamics.

The rotational axis torque (force) generator uses a position servo asdescribed previously. For a sinusoidal angular position command signalthe mass angular position will be given by θ_(m) SIN (ωt). θ_(m) is thepeak mass angular displacement and ω is the radian frequency ofvibration. The mass angular velocity will be given by θ_(m) (ω) COS(ωt). The mass angular acceleration will be -θ_(m) (ω²) SIN (ωt). Theresulting output torque is then

    T=J(θ.sub.m)(ω.sup.2) SIN (ωt)

where J is the rotational inertia of the reaction mass about thevertical axis. The resulting torsional force at the pipe wall is givenby

    F=T/r

where r is the inside radius of the well pipe wall.

Primary limits on maximum torque (force) output as a function offrequency are imposed by limited stroke, supply pressure, servovalveflow and servovalve dynamics.

The radial axis force generator is a pressure control servo as describedpreviously. A nominal static pressure command signal results in acontrol pressure (P_(o)) common to all three actuators. A sinusoidalpressure command signal added to the nominal command gives a commonoutput pressure of P_(o) +P_(c) SIN (ωt). P_(c) is the peak outputpressure change from nominal (P_(o)) and ω is the radian frequency ofvibration. The resulting force at each of the three interfaces of radialactuator plate and pipe wall is then

    F=P.sub.o (A)+P.sub.c (A) SIN (ωt)

where A is the net area of each radial actuator.

Limits on maximum radial force output as a function of frequency areimposed by supply pressure, servovalve flow and servovalve dynamics.Transfer of radial force to the pipe wall is very direct and notdependent upon a path of force transfer through the clamp.

It is anticipated that the actuator of the invention is capable ofgenerating forces between 1000 and 18,000 newtons (220 to 4000 lbf) andeven higher. A preferred level of power for many applications would befrom 4000 to 18000 newtons and a preferred high range of 12000 to 18000newtons for applications requiring higher power such as a deep VSPsurvey. Generating forces above 18000 newtons will place stresses on thecasing cement near the maximum allowed. Cement logs should be examinedin all cases, particularly when generating forces near 18000 newtons andabove, to ensure the cement is not damaged.

FIG. 6 illustrates a further alternative embodiment of a nondestructivedownhole seismic source 90. The seismic source 90 contains a housing 91including a clamping means (92a and b and 93) driven to engage thewellbore 500 within the formation 100 through hydraulic pistons 92a and92b. A hydraulic pump 88 powers the pistons 92a and 92b and a servovalve95 which oscillates a reaction mass 97 through a piston 96 to form aS_(H) -wave. The torsional movement of the piston 96 to generate theS_(H) -wave is illustrated as 3000.

Of course, any of the above identified embodiments to generate S_(V)-waves, radial P-waves, S_(H) -waves, and P-waves can be incorporatedamong the various embodiments to satisfy particular applicationsrequired for a downhole hydraulic source.

FIG. 8 illustrates both graphically and pictorially the operation of adownhole source 30 of the invention (such as source 10 of FIG. 1, source30 of FIG. 2, source 60 of FIG. 4, or source 90 of FIG. 6) within awellbore to generate seismic waves. A logging truck 200 containing acable 300 suspends source 30 into a wellbore 500 through an appropriaterig 400. Graphic pictorial illustration demonstrates the radial S_(V)-waves and P-wave components of a reaction mass operatingreciprocatingly within the wellbore illustrated as 1000. The movement ofthe reaction mass generates S_(V) -waves and a weaker P-wave component.

FIG. 9 illustrates pictorially and graphically the P-wave and S-wavecomponents of a downhole source 30 of the invention (such as source 10of FIG. 1, source 30 of FIG. 2, source 60 of FIG. 4, or source 90 ofFIG. 6) when operated so as to generate a predominantly P-wave in thehorizontal direction (in relation to a vertical wellbore). The reactionforces generated by the source 30 are perpendicular to the wellbore. Themovement is illustrated as 2000 in FIG. 9. The horizontal P-wavecomponent is substantially larger than in FIG. 8.

FIG. 10 pictorially illustrates a cutaway three-dimensional view of anRVSP operation using a downhole source 30 of the invention (such assource 10 of FIG. 1, source 30 of FIG. 2, source 60 of FIG. 4, or source90 of FIG. 6). RSVP can be easily and quickly accomplished with morebeneficial results than VSP through the use of the downhole seismicsource 30 to generate the seismic wave with a multitude of surfacegeophones 675 illustrated as strings of geophones 650a-d connected to arecording truck 600. The logging truck 200 lowers the seismic source 30which is operated to generate the seismic waves of interest, i.e., S_(V)-waves, P-waves or S_(H) -waves, while the geophones record the directand reflected seismic waves (1000/2000). Since the delicate geophones donot have to be lowered into the wellbore environment and a vast array ofsurface geophones that can be utilized, the profiling of the formationcan be done more quickly and efficiently than in VSP.

FIG. 11 illustrates the use of a downhole seismic source 30 of theinvention (such as source 10 of FIG. 1, source 30 of FIG. 2, source 60of FIG. 4, or source 90 of FIG. 6) to perform crosswell tomography. Incrosswell tomography, a first well 400a containing wellbore 500acontains at least one or a plurality of seismic sources, e.g. 30, whichgenerate the desired seismic waves, i.e., S_(V) -waves, S_(H) -waves, orP-waves, directed out into the formation toward at least one receiverwell 400b containing wellbore 500b.

The well 400b contains geophones 650a-n suspended therein but connectedto the appropriate recording truck, not illustrated, on the surface. Asthe seismic source or sources 30 in well 400a generate seismic waves,these waves are recorded by receivers 650a-n within the wellbore 400b togenerate a seismic picture of the geologic formation between wells 400aand 400b. Since the weathering layer normally attenuates seismic wavesgenerated on the surface, surface tomography cannot be carried out forfrequencies greater than about 100 Hz due to the attenuation problems.Of course, the lower the frequency, the less refined the readings arebecause the greater the wavelength of the seismic waves and thus thelower the resolution. Using a downhole seismic source enables thegeneration of high frequency seismic waves, i.e., greater than 100 Hz,to raise the resolution so that smaller and smaller anomalous zoneswithin a given formation can be identified. This can be extremelyvaluable for applications where the producing zones are relatively thinand thus information about them is hard to obtain.

The force output using hydraulic pumps and actuators exceed the forceusing existing pneumatic actuators by a factor of 100 and the forceoutput by heretofore existing electromagnetic actuators by a factor of20-40. The frequency output of the hydraulic actuators is between10-1500 Hz. The maximum frequency of the pneumatic vibrator isapproximately 100-200 Hz. The difference is frequency output is due tothe vast difference in the stiffness between a gas and a fluid.

The motion sensing devices 78 shown in FIG. 1 also enable the presentinvention to operate as a downhole seismic logging tool. The seismicvibrations generated by the actuator 30 travel into the formation 100where they are reflected or refracted back and then detected by thesensing devices 78. By analyzing the signals detected, information aboutthe formation surrounding the wellbore may be deduced. Most surfaceseismic recording is in a range of 10 to 200 hz. The downhole vibratorof the present invention may generate seismic signals at the samefrequency as that recorded from surface seismic sources. Logs obtainedby the present invention using the same frequency as surface generatedseismic data will provide better correlation with surface generatedseismic data than that obtained by existing logging tools which operateat much higher frequencies. The present invention also has greater wavepenetration than existing tools because of its higher power outputcapability.

Three component geophones are preferred for the motion sensors 78.Though FIG. 4 illustrates only a single motion sensor package (76 and78), a series of separately coupled sensor packages is envisioned. Eachsensor package would be acoustically isolated from both the otherseismic sensor packages and the seismic source housing 32.

The invention has been described with reference to particularlypreferred processes of using the downhole seismic source andparticularly preferred embodiments of the hydraulic down seismicvibrator source. Modifications which would be obvious to one of ordinaryskill in the art are contemplated to be within the scope of theinvention.

What is claimed is:
 1. A downhole seismic source comprising:a housing; areaction mass attached to and in said housing; means for clamping thehousing at a predetermined fixed position within a wellbore; and meansfor hydraulically moving said reaction mass to generate at least onedetectable seismic wave of predetermined frequencies attached to saidhousing such that forces are reacted from the reaction mass through thehousing and the means for clamping into the wellbore.
 2. The downholeseismic source according to claim 1 wherein the means for hydraulicallygenerating is a hydraulic actuator capable of generating forces inexcess of about 1000 newtons.
 3. The downhole seismic source accordingto claim 1 wherein the means for hydraulically generating is a hydraulicactuator capable of generating forces of between about 12,000 and 18,000newtons.
 4. The downhole seismic source according to claim 1 wherein theseismic wave has a maximum frequency of between about 10 Hz and 1500 Hz.5. The downhole seismic source according to claim 1 wherein the sourcesweeps the seismic wave from 0 Hz up to a frequency of between about 10Hz and 1500 Hz.
 6. The downhole seismic source according to claim 1wherein the source sweeps the seismic wave from a frequency of between10 Hz and 1500 Hz down to 0 Hz.
 7. The downhole seismic source accordingto claim 1 wherein the means for hydraulically generating is a hydraulicactuator capable of moving the reaction mass in a direction parallel tothe wellbore.
 8. The downhole seismic source according to claim 7wherein the force output of said downhole seismic source is given by therelationship

    F=m(x.sub.m) (ω.sup.2) SIN (ωt)

where F is force output, m is reaction mass, x_(m) is peak massdisplacement, ω is the radian frequency of vibration, and t is time. 9.The downhole seismic source according to claim 1 wherein the means forhydraulically generating is a hydraulic actuator capable of moving thereaction mass in a direction perpendicular to the wellbore.
 10. Thedownhole seismic source according to claim 1 wherein the means forhydraulically generating is a hydraulic actuator capable of moving areaction mass torsionally about its axis which is parallel to thewellbore.
 11. The downhole seismic source according to claim 1 whereinthe force output of said downhole seismic source is given by therelationship

    F=J(θ.sub.m)(ω.sup.2) SIN (ωt) /r

where F is force output, J is rotational inertia of the reaction massabout its vertical axis, θ_(m) is peak mass angular displacement, ω isthe radian frequency of vibration, t is time, and r is inside radius ofthe wellbore.
 12. The downhole seismic source according to claim 1wherein said reaction mass has a motion sensing device attached thereto.13. The downhole seismic source according to claim 1 wherein said meansfor hydraulically generating is a rotational actuator.
 14. The downholeseismic source according to claim 1 wherein said means for clamping iscapable of generating at least one seismic wave by varying the clampingforce it exerts on the wellbore at predetermined frequencies.
 15. Thedownhole seismic source according to claim 14 wherein the force outputof varying the clamping force is given by the relationship

    F=P.sub.o (A)+P.sub.c (A) SIN (ωt)

where F is force output, P_(o) is nominal pressure, on the actuators Ais net area of each hydraulic actuator contacting the wellbore, P_(c) ispeak output pressure change from P_(o), ω is the radian frequency ofvibration, and t is time.
 16. The downhole seismic source according toclaim 14 wherein the seismic wave has a maximum frequency of betweenabout 10 Hz and 1500 Hz.
 17. The downhole seismic source according toclaim 14 wherein the seismic wave can sweep from 0 Hz up to a frequencyof between about 10 Hz and 1500 Hz.
 18. The downhole seismic sourceaccording to claim 14 wherein the seismic wave can sweep from afrequency of between about 10 Hz and 1500 Hz down to 0 Hz.
 19. Thedownhole seismic source according to claim 1 wherein said means forclamping is also capable of generating at least one seismic wave withina formation by:(1) exerting(a) a lower clamping force which is at leastequal to the clamping force necessary to maintain the source at a fixedposition, and (b) a greater clamping force than the lower clampingforce; and (2) alternating between the two clamping forces atpredetermined frequencies.
 20. The downhole seismic source according toclaim 1 wherein the means for moving said reaction mass is furthercomprised of:a hydraulic actuator means for moving the reaction mass ina direction parallel to the wellbore; and a hydraulic actuator means formoving the reaction mass torsionally about its axis which is parallel tothe wellbore.
 21. The downhole seismic source according to claim 1wherein said means for clamping is capable of generating at least oneseismic wave by varying the clamping force it exerts on the wellbore atpredetermined frequencies.
 22. The downhole seismic source according toclaim 1 further comprising at least one motion sensing device, saidmotion sensing device being acoustically isolated from said actuator andcapable of being independently coupled to the wellbore.
 23. The downholeseismic source according to claim 22 wherein said motion sensing devicesare selected from the group consisting of: geophones, accelerometers,and combination thereof.
 24. The downhole seismic source according toclaim 1 further comprising a nonisolated motion sensing device attachedto said housing.
 25. The downhole seismic source according to claim 24wherein said motion sensing devices are selected from the groupconsisting of: geophones, accelerometers, and combinations thereof. 26.The downhole seismic source according to claim 22 further comprising anonisolated motion sensing device, said nonisolated motion sensingdevice being acoustically coupled to said actuator.
 27. The downholeseismic source according to claim 26 wherein said motion sensing devicesare selected from the group consisting of: geophones, accelerometers,and combinations thereof.
 28. The downhole seismic source according toclaim 1 wherein said means for clamping includes clamping means selectedfrom the group consisting of: one hydraulic clamp, two hydraulic clamps,three hydraulic clamps, or four hydraulic clamps, wherein said clampsradially extend out from said housing.
 29. A downhole seismic sourcecomprising:a housing; a reaction mass in said housing: means forclamping the housing at a predetermined position within a wellbore;hydraulic means for moving said mass vertically to generate at least onedetectable seismic wave; hydraulic means for moving said reaction masstorsionally to generate at least one detectable seismic wave; a secondclamping means; hydraulic means for generating forces with said secondclamping means to generate at least one detectable seismic wave.